Buffered SHA1 in Dart with a StreamTransformer

Posted in dart, sha1, streams on September 9, 2013 by Ali

I’ve been hacking somewhat on a Dart client for Camlistore recently, and as part of that I need to take a SHA1 hash of files. This is easy to do in Dart, as the Crypto package takes care of me, and I can produce a simple implementation.

String sha1(List<int> bs) {
  final crypto.SHA1 sha1 = new crypto.SHA1();
  final hexdgst = crypto.CryptoUtils.bytesToHex(sha1.close());
  return 'sha1-${hexdgst}';

Future<String> sha1File(String filename) =>
  new File(filename).readAsBytes().then(sha1);


This is fine until I read a 2 gigabyte file. At that point, I need to buffer the loading. Dart streams to the rescue. You can read a file as a Stream<List<int>> and using a StreamTransformer can just convert that stream into a Stream<String> which is the SHA1.

bufferedSha1Transformer() {
  final sha1 = new crypto.SHA1();
  return new StreamTransformer<List<int>, String>(
    handleData: (List<int> value, EventSink<String> sink) {
    handleDone: (EventSink<String> sink) {
      final dgst = crypto.CryptoUtils.bytesToHex(sha1.close());

Future<String> sha1BufferedFile(String filename) =>
  new File(filename).openRead().transform(bufferedSha1Transformer()).single;


The file is read in chunks of 65536 bytes, and added to the SHA1. When the file is finished the target stream (sink) emits the digest.

Neat, eh? I’ll admit until today I thought Dart streams were annoying, but now I am converted.